The invention relates to polymeric semiconductor die coatings cured with ultra violet light.
Passivation films and other semiconductor die coatings are commonly used as a barrier to physical damage and environmental contaminants. In the manufacture of semiconductor devices, the entire top surface of the wafer is often coated with a passivation film following the formation of the final metal layer. The passivation film is an insulating protective layer that minimizes mechanical and chemical damage to the dies during assembly and packaging. The passivation film material should be impermeable to moisture and to sodium atoms and other highly mobile impurities. Inorganic compounds such as phosphosilicate glass and silicon nitride are often used to form passivation films. More recently, organic polymers have been used for passivation films.
Polyimide is presently the most common type of polymer used to form passivation films. The polyimide film is spun on to the wafer as a liquid polyamic-acid precursor. During high temperature curing, the polyamic acid undergoes a chemical change called imidization that causes it to become the solid polyimide film. Problems with polyimide die coatings center around the ability to properly cure the film. To cure the liquid precursor, high temperature cycles are used to drive the imidization reaction. For example, the wafer is heated to 100xc2x0 C.-150xc2x0 C. for 30 minutes to evaporate the solvent and then the wafer is baked at about 300xc2x0 C. for 60 minutes to fully cure the film. It is not unusual for the polyimide to lose 20%-30% of its volume during the imidization/curing process. The substantial shrinkage of the polyimide film as well as other aspects of the imidization process cause severe stress on the wafer surface which, in extreme cases, leads to film pealing and/or reduction in yield as a result of surface cracks and dislocations induced in the substrate.
Polyimides are also being investigated for use as permanent resist films. Photosensitive polyimide precursors are spun on to the wafer and, upon exposure to ultra violet light, undergo cross linking. During development, the unexposed regions are dissolved and final curing by heat treatment leads to imidization of the remaining crosslinked material. At 275xc2x0 C. nearly all of the precursor is converted to polyimide and most of the photo-crosslinked groups are volatized. The properties of the final cured film are essentially the same as those of the non-photosensitive polyimide. One example of the use of polyimides as a permanent photoresist is described in U.S. Pat. No. 5,013,689 issued to Yamamoto et al. on May 7, 1991. Yamamoto describes a method of forming a two layer passivation film wherein the resist film used to pattern the first passivation film is retained as the second part of the two part passivation film. In Yamamoto, a conventional positive acting resist, a light sensitive polyimide and a light sensitive silicone ladder polymer were each used to pattern a silicon nitride passivation film. The resist was left intact after the passivation film is patterned. It was then xe2x80x9cpost-bakedxe2x80x9d to render it suitable for use as the second part of the resulting two part passivation film.
The present invention is directed to a permanent protective semiconductor die coating made from a polymer that is fully curable through exposure to ultra violet light. A mixture of polymer resin and a photoactive compound is applied to the die and then cured through exposure to ultraviolet light to form the protective coating. In one preferred embodiment, the polymer resin is a phenol-formaldehyde epoxy resin and the photoactive compound is CD1011 (triaryl sulfonium hexafluorophosphate) marketed under the brand name SARTOMER. The coating may be applied as a thin protective film, such as a passivation layer, or as a thicker encapsulant used for semiconductor device packages. Such film coatings exhibit reduced film shrinkage and lower film stresses while maintaining mechanical properties comparable to polyimide film coatings.